1. Field of the invention
The present invention relates to a control apparatus for controlling a motor vehicle having a lean-burn internal combustion engine and a hydraulically operated continuously variable transmission of belt-and-pulley type.
2. Discussion of the Prior Art
For controlling exhaust emission of a motor vehicle, a lean-burn internal combustion engine adapted for combustion of a fuel-lean air-fuel mixture is proposed in JP-A-61-268845. The lean-burn engine disclosed includes an EGR (Exhaust Gas Recirculation) valve, a swirl control valve for causing turbulent flows in the cylinders of the engine, and combustion control means for producing a fuel-lean air-fuel mixture whose air-fuel ratio is higher than the stoichiometric value (A/F=14.5). This lean-burn engine has a plurality of burning conditions, one of which is selected depending upon a load being applied to the engine, so that the engine is operated with reduced amounts of HC, CO and NOx included in exhaust gases. An example of a belt-and-pulley type continuously variable transmission whose speed ratio is continuously variable is disclosed in JP-A-58-191358. The speed ratio of this continuously variable transmission is adjusted so that the internal combustion engine operates according to an optimum fuel consumption curve, assuring improved gas mileage of the vehicle. The above-described lean-burn internal combustion engine also exhibits a desirably high fuel economy due to significantly reduced pumping loss. Accordingly, it is proposed to install the lean-burn engine and continuously variable transmission on the motor vehicle, so as to assure further improved gas mileage of the vehicle.
In a continuously variable transmission having a pair of pulleys and a belt connecting the pulleys as proposed in Japanese Patent Application filed Feb. 1, 1990 in the name of the assignee of the present application, a belt tensioning pressure for controlling the tension of the belt is suitably regulated depending upon an input torque of the transmission, so as to maintain the tension of the belt at a required minimum level. For example, a suitable pressure regulating valve is actuated to produce an optimum belt tensioning pressure, which is determined based on the input torque of the transmission and the effective diameters of the pulleys engaging the belt. The input torque is calculated based on the engine speed and the opening angle of a throttle valve disposed in an intake pipe of the engine, while the effective diameters of the pulleys are calculated based on the current speed-ratio of the transmission. Where the above-described lean-burn engine and continuously variable transmission are both installed on the vehicle, however, the output torque of the engine (i.e., the input torque of the transmission) is inevitably varied depending upon the currently selected burning condition, even with the same engine speed and throttle opening angle. Thus, the known arrangement is incapable of optimally controlling the tension of the belt.